1. Field of the Invention
The present invention generally relates to solid state imagers such as a charge coupled device (CCD) solid state imagers or the like and, more particularly, to a solid state imager in which a micro-condenser lens is formed on a light sensitive portion.
2. Description of the Prior Art
In a solid state imager such as a CCD solid state imager, when considering the relationship among an electrical charge, noise and an illuminance on an image surface on the CCD, it is known that the influences of noise (shot noise) caused by fluctuations of electrical charges and the dark field noise are increased on the low illuminance side.
In order to reduce the above-mentioned shot noise, it is sufficient to increase the aperture efficiency of the light sensitive portion. However, there is a limit to increasing the above aperture efficiency because the solid state imager has recently been more and more miniaturized. To solve this problem, one such structure for the solid state imager is proposed at present, in which a micro-condenser lens is formed on the light sensitive portion. According to this previously-proposed structure in which the micro-condenser lens is formed on the light sensitive portion, the utilization factor of light can be increased, the sensitivity of the light sensitive portion can be increased and the above-mentioned shot noise can be effectively reduced. For example, Japanese Laid-Open Patent Publications Nos. 60-53073 and 1-10666 describe methods of forming a micro-condenser lens.
FIG. 1 shows an example of a structure of a conventional CCD solid state imager. As shown in FIG. 1, a number of transfer electrodes 23 formed of a polycrystalline silicon layer are selectively formed on a silicon substrate 21 through a gate insulating layer 22 made of SiO.sub.2 or the like. An Al light-shielding layer 25 is selectively formed on each of the transfer electrodes 23 through an interlayer 24 and a flattened film 26 made of a resin material is laminated on the whole surface including the Al light-shielding layers 25. Then, a micro-condenser lens 27 is formed on the flattened layer 26. A portion on which the above-mentioned transfer electrode 23 is not formed is provided as a light sensitive portion 28. The Al light-shielding layer 25 is removed on the light sensitive portion 28 and the micro-condenser lens 27 is formed in an opposing relation to the light sensitive portion 28.
There is a limit to which the thickness of the Al light-shielding layer 25 formed on the transfer electrode 23 can be reduced. In addition, since an opening width D of the light sensitive portion 28 is further reduced because the solid state imager is recently more and more miniaturized, an aspect ratio of the thickness of the Al light-shielding film 25 relative to the opening width D of the light sensitive portion 28 tends to increase. Also, since the transfer electrode 23 is formed under the Al light-shielding layer 25, it is frequently observed that the above-mentioned aspect ratio become 1 or more in actual practice. In accordance therewith, a shoulder portion 25a of the Al light-shielding layer 25 on the light sensitive portion 28 protrudes toward the edge of the light sensitive portion 28.
Under this situation, in the conventional CCD solid state imager, the condenser lens 27 is simply formed on the light sensitive portion 28 through the resin (flattened) layer 26 so that, when considering the central portion of the image portion in which a number of pixels (picture elements) are arrayed in a matrix fashion, light beams Lo incident on the apex portion of the condenser lens 27 and on the portion near the apex portion are converged by the condenser lens 27 and thereby are introduced into the light sensitive portion 28, while light beams Le incident on the portion near the circumferential edge of the condenser lens 27 are shielded at the shoulder portion 25a of the Al light-shielding film 25. That is, light components, which are not incident on the light sensitive portion 28, occur and hence a sensitivity increasing effect, which is inherent in the effect of the condenser lens 27, is degraded. This phenomenon frequently occurs in the peripheral portion of the above-mentioned image section so that a peripheral depreciation of light which passes through the camera lens becomes very conspicuous, which unavoidably imposes a limit on making the CCD solid state imager compact in size and on improving the image quality.